A typical aromatic hydrocarbon stream found in petrochemical plants and refineries, such as may be produced by reforming and cracking naphtha, includes the C8 aromatic hydrocarbon isomers ethylbenzene and the xylene isomers paraxylene, metaxylene, and orthoxylene. Paraxylene is relatively high value as compared with the other isomers because it is used as the main raw material for polyester fibers. Orthoxylene, useful such as for preparing phthalate esters for plasticizers, is relatively more valuable than metaxylene. Unfortunately, an equilibrium mixture of xylenes contains roughly twice as much metaxylene as para- or orthoxylene.
To recover paraxylene preferentially, typically a C8 aromatic hydrocarbon stream is processed through a paraxylene recovery stage, such as an adsorption process (e.g., a Parex™ or Eluxyl™ absorptive separation unit) or crystallization process, to recover a paraxylene-enriched stream and a paraxylene-depleted stream. The paraxylene-depleted stream can then be catalytically isomerized to equilibrium for recycle in the paraxylene recovery loop. Ethylbenzene needs to be removed from the loop and one way to do so is as explained below.
Typically the catalyst used to promote isomerization of a paraxylene-depleted stream comprises a zeolite supported with a metal component of Group 7-10 of the Periodic Table, e.g., platinum or rhenium. In addition to promoting isomerization between xylene isomers, ethylbenzene can be converted to benzene through a dealkylation reaction and subsequent hydrogenation of the coproduct ethylene, in the presence of such catalysts. One of the undesired side reactions is metal-catalyzed ring saturation and another is the production of C9+ aromatic hydrocarbons. Research into increasing the efficiency of the paraxylene recovery loop is very active, and in particular there is constant demand for a better catalyst.
Recent prior art related to isomerization processes includes U.S. Pat. Nos. 6,028,238; 7,247,762; 7,270,792; 7,271,118; 7,626,065; U.S. Patent Publication 2011-0190556; and U.S. application Ser. No. 13/081,351.
Bimetallic catalysts per se are well-known and used in many different refining and petrochemical processes, such as in transalkylation in the manufacture of xylenes and other xylene manufacturing processes.
WO2009034093 (US2010/0217057) teaches a new configuration of ZSM-5 having higher average silica to alumina ratio at the edges of each crystallite than in the center providing reduced xylene losses in ethylbenzene dealkylation, especially when combined with silica as binder and one or more hydrogenation metals selected from platinum, tin, lead, silver, copper, and nickel.
WO2007037866 (US2007/0060470) teaches a catalyst of certain combinations of platinum, tin, acidic molecular sieve and aluminum phosphate binder for isomerization and dealkylation activities with low naphthenes make.
U.S. Pat. No. 7,525,008 teaches isomerization of a C8 aromatic stream with a MTW-type zeolite catalyst containing platinum and optionally tin. The MTW type zeolite has a silica to alumina mole ratio of between about 20:1 and 45:1.
US 2011/0190556 teaches a xylene production process involving transalkylation of a C9+ aromatic hydrocarbon feedstock with a C6 and/or C7 aromatic hydrocarbon feedstock. The feedstocks are contacted in the presence of hydrogen with a catalyst system comprising a first bimetallic catalyst and, downstream thereof, a second bimetallic catalyst. The first bimetallic catalyst comprises a molecular sieve having a Constraint Index in the range of 3 to 12, such as ZSM-5, and the metals are selected from Groups 6 to 12 of the Periodic Table. The second bimetallic catalyst comprises a molecular sieve having a Constraint Index less than 3, such as ZSM-12, and the metals are selected from Groups 6 to 12 of the Periodic Table.
US 2010/0048381 teaches a catalyst for xylene isomerization including a carrier having a zeolite with a specified molar ratio of silica to alumina impregnated with or mixed with a metal salt, the carrier supported with a Group VIII metal, or a Group VII metal additionally supported with tin, bismuth, or lead.
US 2007/0060470 teaches a catalyst comprising platinum and tin for the isomerization of xylenes and dealkylation of ethylbenzene. A metal-containing molecular sieve having a silica to alumina ratio of at least 20:1 is taught.
Additional references of interest include U.S. Pat. No. 7,271,118 (WO 2006/022991); U.S. Pat. No. 7,199,070 (EP 1495805); U.S. Pat. Nos. 5,689,027; 5,283,385; 4,485,185; and EP 2022564.
However, none of the systems described above are concerned with processing paraxylene-depleted feed streams through multiple bed systems where the functions of dealkylation and isomerization can be separately managed for improved performance. Furthermore, increased metal selectivity (desired ethylene saturation versus undesired ring saturation), decreased metal migration and decreased susceptibility to sulfur poisoning are still sought after.
The present inventors have surprisingly discovered a bimetallic system for a dealkylation and isomerization of a paraxylene-depleted C8 aromatic hydrocarbon feed stream having low levels of at least one Group 8-10 metal and at least one additional metal that provides, in embodiments, high ethylene saturation and low xylene loss at low temperatures when compared with current state of the art catalysts.